Update to xnu-2422.90.20
Resolve Xcode 6 update warnings (again)
Remove unneeded targets

They will be added back later.
Move build-time scripts to separate files

This makes version management of the build-time scripts infinitely easier.
Remove obsoleted scheme files
Addendum to 2b1415dad
Fix location of availability.pl
Remove -Werror

The Xcode 6 upgrade caused a few dozen new warnings, each of which would be far too time-consuming to track down individually. This change simply prevents the new warnings from breaking the build.